The Oakville radar controller was distracted from the evolving conflict situation by the various demands on his attention created by the requirement for him to perform both the radar and data controller duties during a busy time. Those combined duties are manageable but are routinely performed by radar controllers only during periods of reduced traffic. However, the extra workload created by near simultaneous receipt of estimate-hand-offs and the requirement to change a number of SSRcode assignments for aircraft entering his sector was enough to capture the controller's attention long enough to permit the potential conflict to develop into a loss of separation. Only the TCAS warning to the crew of the involved aircraft prevented a possible mid-air collision.Analysis The Oakville radar controller was distracted from the evolving conflict situation by the various demands on his attention created by the requirement for him to perform both the radar and data controller duties during a busy time. Those combined duties are manageable but are routinely performed by radar controllers only during periods of reduced traffic. However, the extra workload created by near simultaneous receipt of estimate-hand-offs and the requirement to change a number of SSRcode assignments for aircraft entering his sector was enough to capture the controller's attention long enough to permit the potential conflict to develop into a loss of separation. Only the TCAS warning to the crew of the involved aircraft prevented a possible mid-air collision. The Oakville sector radar controller did not detect that the routes of AAL183 and AAL581, at the same altitude, conflicted until advised by the crew of AAL183 of the TCAS RA. By that time, the standard instrument flight rules separation had been lost. The Oakville sector radar controller was performing the radar controller duties as well as the majority of the duties usually performed by the sector data controller. This contributed to the heavy workload and diverted the controller's attention away from the radar display. The radar controller's need to deal with the extra work created by the estimate-hand-offs, in addition to his other duties, captured the controller's attention for a prolonged period time which reduced his ability to detect the impending conflict between AAL183and AAL581.Findings as to Causes and Contributing Factors The Oakville sector radar controller did not detect that the routes of AAL183 and AAL581, at the same altitude, conflicted until advised by the crew of AAL183 of the TCAS RA. By that time, the standard instrument flight rules separation had been lost. The Oakville sector radar controller was performing the radar controller duties as well as the majority of the duties usually performed by the sector data controller. This contributed to the heavy workload and diverted the controller's attention away from the radar display. The radar controller's need to deal with the extra work created by the estimate-hand-offs, in addition to his other duties, captured the controller's attention for a prolonged period time which reduced his ability to detect the impending conflict between AAL183and AAL581. The provision of attentive flight monitoring to active traffic on the radar display is jeopardized when a controller is performing the duties of both the data controller and the radar controller. While efforts have been made by west high supervisors to ensure compliance by Cleveland centre controllers with coordination requirements, estimate-hand-offs continue to occur and can pose a significant additional distraction.Findings as to Risk The provision of attentive flight monitoring to active traffic on the radar display is jeopardized when a controller is performing the duties of both the data controller and the radar controller. While efforts have been made by west high supervisors to ensure compliance by Cleveland centre controllers with coordination requirements, estimate-hand-offs continue to occur and can pose a significant additional distraction. Since this occurrence, conflict alert has become operational in TorontoACC at 14000feet above sea level and above, excluding terminal airspace. NAV CANADA has added one controller on the day shift to avoid the situation in which one controller works more than one data board. Toronto ACC and Cleveland ARTCC (Air Route Traffic Control Center) held discussions which resulted in the staffing of additional full-time day and evening data controllers in both units to manually pass hand-off data.Safety Action Since this occurrence, conflict alert has become operational in TorontoACC at 14000feet above sea level and above, excluding terminal airspace. NAV CANADA has added one controller on the day shift to avoid the situation in which one controller works more than one data board. Toronto ACC and Cleveland ARTCC (Air Route Traffic Control Center) held discussions which resulted in the staffing of additional full-time day and evening data controllers in both units to manually pass hand-off data.